Representing and reconstructing high dynamic range images

ABSTRACT

A high dynamic range image can be recovered from a full-resolution lower-dynamic-range image and a reduced-resolution higher-dynamic-range image. Information regarding higher spatial frequencies may be obtained by extracting high spatial frequencies from the lower-dynamic-range image. In some embodiments an approximate impulse-response function is determined by comparing the higher- and lower-dynamic range images. A scaling image obtained by applying the impulse-response function to a high-frequency band of the lower-dynamic range image is combined with an upsampled higher-dynamic range image to yield a reconstructed image.

TECHNICAL FIELD

The invention relates to high dynamic range digital images. Theinvention relates specifically to methods and apparatus for encoding anddecoding high dynamic range images and to data structures for containingdata representing high dynamic range images.

BACKGROUND

Human vision is capable of appreciating contrast ratios of up to1:10,000. That is, a person can take in a scene in which some parts ofthe scene are 10,000 times brighter than other parts of the scene andsee details in both the brightest and darkest parts of the scene.Further, human vision can adapt its sensitivity to brighter or darkerscenes over a further 6 orders of magnitude.

Most conventional digital image formats (so-called 24-bit formats) useup to 24 bits to store color and luminance information for each pixel inan image. For example, each of a red, green and blue (RGB) value for apixel may be stored in one byte (8 bits). Such formats are capable ofrepresenting brightness variations over only about two orders ofmagnitude (each byte can store one of 256 possible values). There exista number of standard formats for representing digital images (whichinclude both still and video images). These include JPEG (JointPhotographic Experts Group), MPEG (Motion Picture Experts Group), AVI(Audio Video Interleave), TIFF (Tagged Image File Format), BMP (BitMap), PNG (Portable Network Graphics), GIF (Graphical InterchangeFormat), and others. Such formats may be called “output referredstandards” because they do not attempt to preserve image informationbeyond what can be reproduced by electronic displays of the types mostcommonly available. Until recently, displays such as computer displays,televisions, digital motion picture projectors and the like have beenincapable of accurately reproducing images having contrast ratios betterthan 1:1,000 or so.

Display technologies being developed by the assignee, and others, areable to reproduce images having high dynamic range (HDR). Such displayscan reproduce images which more faithfully represent real-world scenesthan conventional displays. There is a need for formats for storing HDRimages for reproduction on these displays and other HDR displays thatwill become available in the future.

A number of formats have been proposed for storing HDR images as digitaldata. These formats all have various disadvantages. A number of theseformats yield prohibitively large image files that can be viewed onlythrough the use of specialized software. Some manufacturers of digitalcameras provide proprietary RAW formats. These formats tend to becamera-specific and to be excessive in terms of data storagerequirements.

There is a need for a convenient framework for storing, exchanging, andreproducing high dynamic range images. There is a particular need forsuch a framework which is backwards-compatible with existing imageviewer technology. There is a particular need for backwardscompatibility in cases where an image may need to be reproduced bylegacy devices, such as DVD players, which have hardware-based imagedecoders.

Some related publications include:

-   -   Ward, Greg, A General Approach to Backwards-Compatible Delivery        of High Dynamic Range Images and Video” Proceedings of the        Fourteenth Color Imaging Conference, November 2006.    -   Rafal Mantiuk, Grzegorz Krawczyk, Karol Myszkowski, Hans-Peter        Seidel, Perception-motivated High Dynamic Range Video Encoding,        Proc. of SIGGRAPH '04 (Special issue of ACM Transactions on        Graphics).    -   Rafal Mantiuk, Alexander Efremov, Karol Myszkowski, Hans-Peter        Seidel, Backward Compatible High Dynamic Range MPEG Video        Compression, Proc. of SIGGRAPH '06 (Special issue of ACM        Transactions on Graphics).    -   Greg Ward & Maryann Simmons, Subband Encoding of High Dynamic        Range Imagery, First Symposium on Applied Perception in Graphics        and Visualization (APGV).    -   Greg Ward & Maryann Simmons, JPEG-HDR: A Backwards-Compatible,        High Dynamic Range Extension to JPEG, Proceedings of the        Thirteenth Color Imaging Conference.    -   U.S. Pat. No. 4,649,568.

The foregoing examples of the related art and limitations relatedthereto are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those of skill inthe art upon a reading of the specification and a study of the drawings.

SUMMARY

The following embodiments and aspects thereof are meant to be exemplaryand illustrative, not limiting in scope.

One aspect of the invention provides apparatus for reconstructing ahigher-dynamic-range image. The apparatus comprises: a spatial filteringstage configured to process at least a full resolution, lower dynamicrange (FRLDR) representation of an image to obtain a scaling imagecomprising information relating to high spatial frequency components ofthe FRLDR representation; an upsampling stage configured to upsample alower resolution, higher-dynamic-range (LRHDR) representation of theimage, to obtain an upsampled image; and a combining stage configured tocombine the scaling image and the upsampled image to obtain areconstructed higher-dynamic-range image. The LRHDR representation has alower resolution and a higher dynamic range than the FRLDRrepresentation.

Another aspect of the invention provides a method for reconstructing ahigher-dynamic-range image, The method comprises obtaining a FRLDRrepresentation of the image and a LRHDR representation of the image, TheLRHDR representation has a lower resolution and a higher dynamic rangethan the FRLDR representation. The method processes at least the FRLDRrepresentation to obtain a scaling image comprising information relatingto high spatial frequency components of the FRLDR representation andupsamples the LRHDR representation to obtain an upsampled image. Themethod combines the scaling image and the upsampled image to obtain areconstructed higher-dynamic-range image.

Further aspects of the invention and features of embodiments of theinvention are described below and illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

Exemplary embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be considered illustrative rather than restrictive.

FIG. 1 shows a data structure according to an embodiment of theinvention.

FIG. 2 is a flow chart illustrating a general method for obtaining theFIG. 1 data structure according to a particular embodiment of theinvention.

FIG. 3 is a flow chart illustrating a method for recoveringhigh-dynamic-range (HDR) data from the FIG. 1 data structure accordingto a basic embodiment of the invention.

FIG. 3A is a flow chart illustrating one method for obtaining a scalingimage of the type used in the method of FIG. 3.

FIG. 4 is a flow chart illustrating a method for recoveringhigh-dynamic-range (HDR) data from the FIG. 1 data structure accordingto a more detailed embodiment of the invention.

FIG. 4A is a flow chart illustrating a method for obtaining a localimpulse-response function of the type used in the method of FIG. 4.

FIG. 4B is a schematic view of a portion of an image.

FIG. 5 is a plot illustrating an approximate local impulse-responsefunction.

FIG. 6 illustrates apparatus according to an embodiment of theinvention.

FIG. 7 illustrates a medium according to an embodiment of the invention.

DESCRIPTION

Throughout the following description specific details are set forth inorder to provide a more thorough understanding to persons skilled in theart. However, well known elements may not have been shown or describedin detail to avoid unnecessarily obscuring the disclosure. Accordingly,the description and drawings are to be regarded in an illustrative,rather than a restrictive, sense.

One aspect of this invention provides a data format for representinghigh-dynamic-range (HDR) images that includes a full-resolution,lower-dynamic-range (FRLDR) image and a lower-resolution, buthigher-dynamic-range (LRHDR) image. The (FRLDR) image can be encoded ina way that provides backwards compatibility with existing image formats.The LRHDR image can be used in conjunction with the FRLDR image, asdescribed herein, to provide a full resolution HDR image.

FIG. 1 shows a data structure 10 that includes a full-resolution,lower-dynamic-range (FRLDR) image 12 and a lower-resolution, buthigher-dynamic-range (LRHDR) image 14. By way of example only, FRLDRimage 12 may comprise data representing an image in a 24-bit format.LRHDR image 14 may comprise data representing the same image in a formathaving a higher dynamic range than that of FRLDR image 12.

Data structure 10 may be readable by legacy image viewers. The legacyimage viewers may read data defining FRLDR image 12 and ignore datarepresenting LRHDR image 14.

Each of images 12 and 14 may be compressed in any suitable manner. Thesame or different compression methods may be used to compress each ofimages 12 and 14. In one embodiment, data structure 10 has a format thatprovides main image data and an auxiliary stream andhigher-dynamic-range image 14 is in the auxiliary stream. For example,data structure 10 may comprise a JFIF file and FRLDR image 12 maycomprise a JPEG image. In some embodiments, data structure 10 comprisesa MPEG file and FRLDR image 12 comprises a frame of a MPEG video.

The resolution of LRHDR image 14 is lower than that of FRLDR image 12 inat least one dimension and preferably in both dimensions. For example,the resolution of LRHDR image 14 may be ¼ of that of FRLDR image 12 inboth dimensions (so that LRHDR image 14 contains 1/16th as many pixelsas FRLDR image 12).

Data structure 10 may be encoded as shown in FIG. 2. Originalhigh-dynamic-range (HDR) data 15 is encoded in block 16 to produce LRHDRimage 14. Original lower-dynamic-range (LDR) image 18 is encoded inblock 19 to yield FRLDR image 12. Original LDR data 18 may be derivedfrom original HDR data 15, for example, by applying a tone-mappingoperator as illustrated in optional block 17. In the alternative,original LDR data 18 may be obtained separately or derived in adifferent manner from original HDR data 15.

Basic and more-advanced methods for reconstructing a full-resolution,high-dynamic-range (FRHDR) image from FRLDR image 12 and LRHDR image 14will now be described. A basic reconstruction method 20 is shown in FIG.3.

Method 20 decodes FRLDR image 12 and LRHDR image 14, if necessary, inblocks 22 and 32. From FRLDR image 12, method 20 obtains a scaling image30 comprising information regarding components of FRLDR image 12 havinghigher spatial frequencies. In some embodiments, the pixels of scalingimage 30 have an average value of 1.0.

In some embodiments, method 20 includes a block 23 that inverts thetone-mapping curve used in the creation of FRLDR image 12 (see block 17of FIG. 2). Processing FRLDR image 12 using such inverted tone mappingcan yield improved results (e.g. a better quality recovered FRHDR image)by recovering full contrast in the high frequencies of scaling image 30.In some cases the tone-mapping operator used in the creation of FRLDRimage 12 may be unknown or it may be undesirable to incur the addedoverhead of inverting the tone-mapping curve.

Method 20 converts FRLDR image 12 into the same color space as LRHDRimage 14, if necessary, in block 24. Method 20 then extracts highspatial frequency components from the decoded and color-converted FRLDRimage 12 in block 26 to yield scaling image 30. Preferably, block 26extracts those spatial frequencies that are present in FRLDR image 12,but are not present in LRHDR image 14 because of the lower resolution ofLRHDR image 14.

In some embodiments, block 26 extracts spatial frequencies that areabove a quantization threshold of FRLDR image 12. The quantizationthreshold may be set high enough that artifacts that arise from the factthat the pixel values of FRLDR image 12 change in discrete steps areignored.

Method 20 also involves upsampling the decoded LRHDR image 14 in block34 to the same resolution as scaling image 30 and FRLDR image 12 toyield an upsampled image 38.

Upsampled image 38 is combined with scaling image 30 at block 40 toyield a recovered full resolution, high-dynamic-range (FRHDR) image 42.In the illustrated embodiment, the pixels of scaling image 30 have anaverage value of 1.0 and combining upsampled image 38 with scaling image30 comprises multiplying pixel values from upsampled image 38 withcorresponding pixel values from scaling image 30.

Block 26 may perform any of various suitable methods for extractinginformation regarding high spatial frequency components of FRLDR image12. For example, block 26 may apply a rational convolution filter to thedata of FRLDR image 12. FIG. 3A illustrates an example method 50 forextracting information regarding high spatial frequency components ofFRLDR image 12. Method 50 operates on the decoded and color-convertedFRLDR image 52 by downsampling in block 54 to yield downsampled image55.

Downsampled image 55 has a resolution low enough that thehigh-frequencies of interest are substantially removed. For example,downsampled image 55 may have a resolution equal to that of LRHDR image14. In block 56, downsampled image 55 is upsampled to yield a resampledimage 57 having a resolution equal to that of FRLDR image 53. Upsamplingblock 56 preferably applies an upsampling algorithm that introduces fewspurious high frequencies. Ideally, spatial frequencies higher than theresolution limit of downsampled image 55 are substantially absent fromresampled image 57. For example, block 56 may perform upsampling using abilinear interpolation to minimize introduction of spurious highfrequencies into resampled image 57.

In block 58, resampled image 57 is combined with FRLDR image 53 to yieldscaling image 30. In the illustrated embodiment, block 58 divides eachpixel value of FRLDR image 53 by the value of a corresponding pixel inresampled image 57.

FIG. 4 illustrates a method 60 for reconstructing high dynamic rangeimages that can be used even in cases where lower-dynamic-range image 12has been created with the use of a tone-mapping operator that is complexor unknown. Method 60 compares characteristics of FRLDR image 12 andLRHDR image 14 to obtain an approximate impulse-response for thetone-mapping operator used in the creation of FRLDR image 12.

If necessary, as described above, in block 62 method 60 converts thecolor space of FRLDR image 12 to be compatible with the color space ofLRHDR image 14 to yield color-corrected FRLDR image 63 that is used asthe basis for further processing.

Comparing FRLDR image 12 and LRHDR image 14 to obtain an approximateimpulse-response is facilitated by providing a version of FRLDR image 12that is equal in resolution to LRHDR image 14. Further, since LRHDRimage 14 lacks the highest spatial frequencies of FRLDR image 12, thereis nothing in LRHDR image 14 to compare to those highest spatialfrequencies. Method 60 obtains an approximate impulse-response bycomparing information regarding at least the highest spatial frequenciesthat are present in both FRLDR image 12 and LRHDR image 14.

In block 66 the resolution of color-corrected FRLDR image 63 is reduced(e.g. by downsampling) to yield reduced-resolution LDR image 67. Highspatial frequencies are extracted from reduced-resolution LDR image 67in block 68 to yield the high-frequency band 69 of reduced-resolutionLDR image 67.

In block 70 high frequencies are extracted from LRHDR image 14 to yieldthe high-frequency band 71 of LRHDR image 14. Block 74 compareshigh-frequency bands 69 and 71 to yield an impulse function 75. One wayto perform this block 74 comparison is described below with reference toFIG. 4A.

In block 78, LRHDR image 14 is upsampled to yield upsampled HDR image 79having the desired resolution. The resolution of upsampled HDR image 79is typically equal to the resolution of FRLDR image 12 and is greaterthan that of LRHDR image 14. Upsampled HDR image 79 lacks informationabout higher spatial frequencies. This information is obtained fromFRLDR image 12 using impulse-response function 75. In block 80, highspatial frequencies are extracted from color-corrected FRLDR image 63 toyield the high frequency band 81 of FRLDR image 12.

In cases where FRLDR image 12 is provided in a format in which ahigher-frequency band of spatial frequencies is stored separately thenhigher frequency bands may optionally be extracted directly from thestored/encoded image format without the requirement of a spatialfiltering or other step for extracting the higher spatial frequencies.

In block 84, impulse function 75 is applied to high-frequency band 81 toyield scaling image 85. A quantization threshold may be applied in block84 to obtain scaling image 85. In such embodiments, impulse function 75is applied to pixels in high frequency band 81 having values at leastequal to (or greater than) the applicable quantization threshold. Insome embodiments, the applicable quantization threshold is determinedfrom a quantization threshold table. In block 88, scaling image 85 iscombined with upsampled HDR image 79 to yield reconstructedfull-resolution, high-dynamic-range (FRHDR) image 89.

In general, impulse-response function 75 can be position-dependent (i.e.dependent on position within an image). Therefore, in preferredembodiments, method 60 determines impulse-response function 75 as afunction of position. FIG. 4A shows a method 90 for obtaining anapproximate local impulse-response function from high-frequency bands 69and 71. Blocks 92A and 92B involve obtaining groups of pixel values fromhigh-frequency bands 69 and 71 respectively. The pixel values used arepreferably luminance (gray) values. The block 92A, 92B groups mayinclude all pixels taken from within a square or rectangular region of ahigh-frequency band 69, 71, for example, although this is not mandatory.The regions may have other shapes. The block 92A, 92B groups may eachinclude all pixel values within a defined region of the correspondinghigh-frequency band 69, 71 or selected pixels. For example, the block92A, 92B groups may include every second, third, or N^(th) pixel inevery second, third, or M^(th) row within the defined region.

Blocks 94A and 94B involve sorting the pixel values from the block 92A,92B groups into lists (e.g. in the order of their luminance values).Block 96 obtains pairs of values that have corresponding index positionsin the block 94A, 94B sorted lists. Block 96 may take all pairs ofvalues but preferably takes a reduced selection of the pairs of values.For example, block 96 may involve obtaining pairs of values comprisingevery p^(th) value from each of the block 94A, 94B sorted lists. In someembodiments, block 96 selects in the range of 20-50 pairs of values thatare spaced-apart between the minimum pixel values (i.e. at one end ofthe block 94A, 94B sorted lists) and the maximum pixel values (i.e. atthe opposing end of the block 94A, 94B sorted lists).

A prototype embodiment obtains block 92A, 92B groups of 4096 pixelvalues (from regions of high frequency bands 69, 71 that are 64 pixelsby 64 pixels), places the pixel values in arrays that are 4096 entireslong, sorts the arrays (blocks 94A, 94B), and then, in block 96, obtainsevery 164^(th) entry from each of the block 94A, 94B sorted arrays toprovide a set of 25 monotonically-increasing coordinate pairs.

It has been found that acceptable results can be obtained by takingpixels within regions that are 64 pixels by 64 pixels or larger in size(e.g. regions containing about 4000 or more pixels).

FIG. 5 is a plot showing the correlation between sorted pixel valuesfrom corresponding regions of high-frequency bands 69 and 71, for a setof example data. In the illustrated embodiment, value pairs selected inblock 96 are indicated by rectangles. A local impulse-response function99 is obtained by fitting a curve or curves (by interpolation (shown inblock 98) or otherwise) to the selected pixel value pairs. By way ofexample, linear or cubic interpolation may be used to obtain anapproximate impulse-response that matches a set of coordinates. In FIG.5, line 97 is an approximate impulse-response obtained by linearinterpolation (block 98) between the block 96 selected pixel valuepairs.

It is desirable to extrapolate the impulse-response functions to dealwith pixel values outside of the range of the pixel value pairs used toderive the impulse-response function. This may be done, for example, atthe upper end by extrapolating from the largest block 96 value pair by adistance that is ½ of the distance between the largest andsecond-largest block 96 value pairs. This may be done at the lower endby extrapolating from the smallest block 96 value pair by a distancethat is ½ of the distance between the smallest and second-smallest block96 value pairs.

In some embodiments, the slope of the impulse-response function iscapped in order to reduce excessive quantization noise in high gradientareas. For example, the input response functions may be scaled, ifnecessary, to reduce the average slope of the impulse-response functionto below a threshold slope. For example, the threshold slope may beabout 1:5.

It would be possible (although very computationally-intensive) todetermine a different local impulse-response function for every pixel.Significant computation can be saved by applying the response functionsof the nearest neighboring blocks and interpolating the results. Thiscan be done with little loss of quality assuming that theimpulse-response function is reasonably slowly-varying with positionwithin an image.

FIG. 4B illustrates one approach to determining impulse-responsefunctions applicable to individual pixels. An image is divided into aplurality of regions 100. In FIG. 4B, the four regions 100 areindividually identified as 100A, 100B, 100C and 100D and region 100A isshaded. In the illustrated embodiment, regions 100 are square. A localimpulse-response function is determined for each region 100, asdescribed above. The impulse-response function is associated with arepresentative pixel 102 of the region 100. Representative pixels 102are preferably located centrally in their corresponding regions 100.FIG. 4B shows four representative pixels 102A, 102B, 102C and 102D, eachassociated with a corresponding one of regions 100A, 100B, 100C and100D.

As seen in FIG. 4B, regions 100 overlap with one another. In theillustrated embodiment, regions 100 overlap adjoining regions by 50%, sothat all pixels away from the edges of the image belong to four regions.Preferably, regions 100 overlap with adjacent regions by at least 25%.The impulse-response function for a pixel 103 having an arbitrarylocation may be determined from the local impulse-response functionscorresponding to the one or more representative pixels 102 that areclosest to the pixel 103.

In the illustrated case, pixel 103 is located between representativepixels 102A, 102B, 102C and 102D (i.e. pixel 103 is overlapped byregions 100A, 100B, 100C, 100D). An impulse-response functionappropriate to pixel 103 may be obtained by interpolating theimpulse-response functions corresponding to representative pixels 102A,102B, 102C and 102D based upon the distances between pixel 103 and eachof pixels 102A, 102B, 102C and 102D. The value of pixel 103 may beapplied as an input to the impulse-response functions corresponding toeach of representative pixels 102A, 102B, 102C and 102D. The fourresults may be combined to provide a result appropriate to pixel 103 bylinear interpolation.

FIG. 6 shows apparatus 200 according to an embodiment of the invention.In some embodiments, apparatus 200 is integrated with a DVD player,computer display, video player, television, or other image-displayingapparatus. Apparatus 200 comprises a spatial filtering stage 204 thatreceives FRLDR image data 12 and processes the FRLDR image data 12 toyield high frequency scaling image 30. Apparatus 200 also comprisesupsampling stage 206 that receives LRHDR image data 14 and upsamples toobtain upsampled image 38. A combining stage 208 combines scaling image30 and upsampled image 38 to yield a reconstructed image data 42.Apparatus 200 optionally includes a display 209 which displaysreconstructed image data 42. Stages 204, 206 AND 208 of apparatus 200may be implemented in suitable image-processing hardware or softwarebeing executed on a suitable data processor or on combinations thereof.

Some embodiments of the invention comprise media having both a fullresolution, lower dynamic range (FRLDR) representation of an image and alower resolution, higher-dynamic-range (LRHDR) representation of theimage. The LRHDR representation has a lower resolution and a higherdynamic range than the FRLDR representation. The FRLDR representationmay be viewed on lower-dynamic-range displays. The LRHDR representationmay be used together with the FRLDR representation as described above toobtain full resolution higher-dynamic-range images for display onhigher-dynamic range displays. FIG. 7 shows a medium 210 (which could,for example, comprise a DVD, magnetic storage device, flash RAM, CD orthe like) which includes both a FRLDR representation 212 of an image anda LRHDR representation 214 of the same image. In some embodiments theFRLDR representation of the image has at least 16 times more pixels thanthe LRHDR representation.

Certain implementations of the invention comprise computer processorswhich execute software instructions which cause the processors toperform a method of the invention. For example, one or more processorsin a DVD-player, computer, television, data projector, or otherimage-displaying computerized device may implement the methods of theinvention by executing software instructions in a program memoryaccessible to the processors. The invention may also be provided in theform of a program product. The program product may comprise any mediumwhich carries a set of computer-readable signals comprising instructionswhich, when executed by a data processor, cause the data processor toexecute a method of the invention. Program products according to theinvention may be in any of a wide variety of forms. The program productmay comprise, for example, media such as magnetic data storage mediaincluding floppy diskettes, hard disk drives, optical data storage mediaincluding CD ROMs, DVDs, electronic data storage media including ROMs,flash RAM, or the like. Program products may be distributed by way oftransmission-type media such as digital or analog communication links.The software instructions on a program product may be optionallycompressed and/or encrypted.

Where a component (e.g. a software module, processor, assembly, device,circuit, etc.) is referred to above, unless otherwise indicated,reference to that component (including a reference to a “means”) shouldbe interpreted as including as equivalents of that component anycomponent which performs the function of the described component (i.e.,that is functionally equivalent), including components which are notstructurally equivalent to the disclosed structure which performs thefunction in the illustrated exemplary embodiments of the invention.

While a number of exemplary aspects and embodiments have been discussedabove, those of skill in the art will recognize certain modifications,permutations, additions and sub-combinations thereof. It is thereforeintended that the following appended claims and claims hereafterintroduced are interpreted to include all such modifications,permutations, additions and sub-combinations as are within their truespirit and scope.

1. A computer-implemented method for reconstructing ahigher-dynamic-range image, the method comprising: obtaining a fullresolution, lower dynamic range (FRLDR) representation of the image anda lower resolution, higher-dynamic-range (LRHDR) representation of theimage, the LRHDR representation having a lower resolution and a higherdynamic range than the FRLDR representation; and in a data processor:processing at least the FRLDR representation to obtain a scaling imagecomprising information relating to high spatial frequency components ofthe FRLDR representation; upsampling the LRHDR representation to obtainan upsampled image; and combining the scaling image and the upsampledimage to obtain a reconstructed higher-dynamic-range image.
 2. A methodaccording to claim 1 wherein the FRLDR representation is created from anoriginal full resolution, high-dynamic-range representation of the imageusing a tone-mapping operation and wherein processing at least the FRLDRrepresentation comprises inverting the tone-mapping operation.
 3. Amethod according to claim 1 wherein processing at least the FRLDRrepresentation to obtain the scaling image comprises: downsampling theFRLDR representation to obtain a downsampled image wherein thedownsampled image has a resolution sufficiently low to comprise onlyspatial frequency components below a desired spatial frequency level;upsampling the downsampled image to obtain a resampled image; andcombining the resampled image and the FRLDR representation to obtain thescaling image.
 4. A method according to claim 3 wherein combining theresampled image and the FRLDR representation comprises dividing eachpixel value of the FRLDR representation by a corresponding pixel valueof the resampled image.
 5. A method according to claim 3 wherein anaverage of pixel values in the scaling image is
 1. 6. A method accordingto claim 1 wherein combining the scaling image and the upsampled imagecomprises multiplying each pixel value of the scaling image with acorresponding pixel value of the upsampled image.
 7. A method accordingto claim 1 wherein processing at least the FRLDR representation toobtain the scaling image comprises applying a convolution filter to theFRLDR representation.
 8. A method according to claim 1 whereinprocessing the FRLDR representation comprises converting a color spaceof the FRLDR representation into a color space of the LRHDRrepresentation.
 9. A method according to claim 1 wherein processing atleast the FRLDR representation to obtain the scaling image comprises:using the FRLDR representation and the LRHDR representation to determinean impulse-response function; extracting high spatial frequencycomponents of the FRLDR representation; and multiplying each pixel valueof the extracted high spatial frequency components by a correspondingvalue of the impulse-response function to determine the scaling image.10. A method according to claim 9 comprising applying a quantizationthreshold to the high spatial frequency components.
 11. A methodaccording to claim 10 wherein using the FRLDR representation and theLRHDR representation to determine an impulse-response functioncomprises: extracting high spatial frequency components of the LRHDRrepresentation to obtain a high frequency band of the LRHDRrepresentation; reducing a resolution of the FRLDR representation toobtain a reduced resolution low-dynamic-range image; extracting highspatial frequency components of the reduced resolution low-dynamic-rangeimage to obtain a high frequency band of the reduced resolutionlow-dynamic-range image; and processing the high frequency band of theLRHDR representation and the high frequency band of the reducedresolution low-dynamic-range image to obtain the impulse-responsefunction.
 12. A method according to claim 11 wherein processing the highfrequency band of the LRHDR representation and the high frequency bandof the reduced resolution low-dynamic-range image to obtain theimpulse-response function comprises: (a) selecting a group of pixelsfrom within a spatial region of the high frequency band of the LRHDRrepresentation and selecting a corresponding group of pixels from withinthe spatial region of the high frequency band of the reduced resolutionlow-dynamic-range image; (b) sorting the group of pixels and thecorresponding group of pixels by pixel value to obtain a pair of sortedarrays defining a regional impulse-response function corresponding tothe spatial region; (c) combining one or more regional impulse-responsefunctions to obtain the impulse-response function.
 13. A methodaccording to claim 12 wherein combining one or more regionalimpulse-response functions comprises repeating steps (a) through (b) fora plurality of spatial regions.
 14. A method according to claim 12wherein step (b) comprises selecting a plurality of pairs of pixelvalues from the pair of sorted arrays, each pair of pixel valuescomprising pixel values having a common index within the pair of sortedarrays; and, interpolating between the plurality of pairs of pixelvalues.
 15. A method according to claim 12 comprising determining animpulse-response function value for a pixel by determining a regionalimpulse-response function value for each of a plurality of regions thatinclude the pixel and computing a weighted combination of the regionalimpulse-response function values.
 16. A method according to claim 15comprising computing the weighted combination of the regionalimpulse-response function values comprises weighting the regionalimpulse-response function values based on distances of the pixel fromcentral points in the regions to which the regional impulse-responsefunction values correspond.
 17. A method according to claim 14 whereinstep (b) comprises extrapolating the regional impulse-response functionpast at least one of the smallest and largest ones of the pairs of pixelvalues.
 18. Apparatus for reconstructing a higher-dynamic-range image,the apparatus comprising: a data processor; a spatial filtering stageconfigured to process at least a full resolution, lower dynamic range(FRLDR) representation of an image to obtain a scaling image comprisinginformation relating to high spatial frequency components of the FRLDRrepresentation; an upsampling stage configured to upsample a lowerresolution, higher-dynamic-range (LRHDR) representation of the image, toobtain an upsampled image; the LRHDR representation having a lowerresolution and a higher dynamic range than the FRLDR representation; anda combining stage configured to combine the scaling image and theupsampled image to obtain a reconstructed higher-dynamic-range image,wherein the spatial filtering stage, upsampling stage and combiningstage each comprise computer-readable software instructions that areexecutable by the data processor, the computer-readable instructionsstored in a program product comprising a tangible computer-readablemedium.
 19. Apparatus according to claim 18 comprising a display whereinthe apparatus is configured to display the reconstructed higher-dynamicrange image on the display.